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United States Patent |
6,043,539
|
Sugasawara
|
March 28, 2000
|
Electro-static discharge protection of CMOS integrated circuits
Abstract
In a semiconductor integrated circuit, I/O buffer circuits that include ESD
protection are generally provided for each I/O pad. According to the
invention, unused pads, i.e. pads that are not connected to core circuitry
according to an initial design, are connected to other pads that are used
for connection to the core circuitry, thereby employing the unused pads to
improve ESD protection of susceptible pads. This approach has the
advantages of greater ESD protection without increasing silicon area and
without adding any additional steps to the usual fabrication process. The
inventive concept is especially useful for augmenting ESD protection of
corner pads without requiring new or custom ESD protection circuits. This
invention can be easily implemented into known layout tools.
Inventors:
|
Sugasawara; Emery (Pleasanton, CA)
|
Assignee:
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LSI Logic Corporation (Milpitas, CA)
|
Appl. No.:
|
978979 |
Filed:
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November 26, 1997 |
Current U.S. Class: |
257/357; 257/355; 257/786; 361/220 |
Intern'l Class: |
H01L 023/60; H01L 023/62 |
Field of Search: |
257/355,356,357,786
361/56,57,90,93,100,101,220
|
References Cited
U.S. Patent Documents
5410254 | Apr., 1995 | Consiglio | 324/456.
|
5712753 | Jan., 1998 | Yeh et al. | 361/56.
|
5715127 | Feb., 1998 | Yu | 361/56.
|
5818086 | Oct., 1998 | Lin et al. | 257/355.
|
Foreign Patent Documents |
62-230028 | Oct., 1987 | JP | 257/786.
|
Other References
Matsumoto, et al., "New Failure Mechanism due to Non-Wired Pin ESD
Stressing," EOS/ESD Symposium, 1994, pp. 2.5.1-2.5.6.
|
Primary Examiner: Guay; John
Claims
I claim:
1. A semiconductor integrated circuit product comprising:
a circuit chip containing core electronic circuitry and having at least
first and second I/O pads;
at least two buffer structures formed on the circuit chip, one of the
buffer structures being connected to the first pad and the other buffer
structure being connected to the second pad;
the buffer structures each including an ESD protection structure for
protecting core electronic circuitry connected to the corresponding pad
from excessive voltage appearing at a corresponding external pin; and
a conductor interconnecting the first pad and the second pad, thereby
interconnecting the corresponding buffer structures to one another in
parallel so as to improve ESD protection of the core circuitry connected
to the first pad,
wherein the first pad is directly coupled with the core electronic
circuitry and the second pad is coupled with the core electronic circuitry
only through the conductor.
2. A semiconductor integrated circuit product according to claim 1,
wherein:
the pads are located along a first peripheral edge of the chip; and
the first pad and the second pad are located adjacent one another along the
first peripheral edge of the chip.
3. A semiconductor integrated circuit product according to claim 1,
wherein:
the chip includes pads arranged along generally rectilinear edges of the
chip including first and second adjacent edges forming a corner of the
chip; and
the first pad is located adjacent the corner of the chip.
4. A semiconductor integrated circuit product according to claim 1, further
comprising a second conductor interconnecting the first pad and a third
pad of the circuit chip, thereby interconnecting the corresponding buffer
structures to one another in parallel so as to further improve ESD
protection of the core circuitry connected to the first pad,
wherein the third pad is coupled with the core electronic circuitry only
through the second conductor.
5. A semiconductor integrated circuit product according to claim 1,
wherein:
the product is an ASIC product; and
each of the buffer structures is a standard cell buffer circuit.
6. A semiconductor integrated circuit product according to claim 1, wherein
the conductor comprises a metallization strap extending between the second
pad and the first pad.
7. A method of improving ESD protection of core circuitry connected to a
first I/O pad in a semiconductor integrated circuit, the method comprising
the steps of:
providing a buffer circuit having an ESD protection structure connected to
the first I/O pad;
identifying an unused pad on the IC that is not connected to the core
circuity but includes an ESD protection structure connected to the pad;
and
electrically connecting the unused pad to the first pad, thereby connecting
the corresponding ESD protection structures to each other in parallel, so
that improved ESD protection is provided to the first I/O pad without
adding additional circuitry to the chip.
8. A method according to claim 7 wherein said connecting step comprises
forming a conductive structure between the used pad and the unused pad as
part of a metallization step in which other conductive structures are
formed interconnecting one or more of the pads to the core circuitry
whereby no special fabrication step is necessary to employ the unused
buffer structures.
9. A method according to claim 7 wherein the identifying step comprises
identifying an unused pad immediately adjacent to the first pad.
10. A method according to claim 7 wherein the identifying step comprises
first identifying an I/O pad susceptible to ESD damage and designating the
susceptible pad as the first pad; and then identifying the unused pad as
an unused pad nearest to the first pad.
11. A method of designing an ASIC for improved ESD immunity, the ASIC
including an array of I/O pads generally for connecting core circuitry to
external package pins, each I/O pad including a standard I/O buffer, and
each I/O buffer including an ESD protection structure, the method
comprising the steps of: obtaining an initial design of the ASIC including
I/O connections;
determining which of the I/O pads in the array are not used for connection
to the core circuitry according to the initial design; and
modifying the initial design so as to provide interconnection of at least
one of the unused pads to a selected pad in the array that is used for
connection to the core circuitry, thereby enhancing ESD protection of the
core circuitry connected to the selected pad.
12. A method of designing an ASIC according to claim 11 wherein the
selected pad is adjacent a corner of the ASIC chip.
13. A method of designing an ASIC according to claim 11 wherein the said
one of the unused pads is immediately adjacent to the selected pad.
14. A method of designing an ASIC according to claim 11 wherein said
modifyig step includes modifying a metal layer in the initial design of
the ASIC, whereby no extra processing steps are required to connect the
unused pad to the selected pad during fabrication.
15. A method of designing an ASIC according to claim 11 wherein the unused
pad is spaced apart from the selected pad.
Description
FIELD OF THE INVENTION
The present invention is related to ESD protection in integrated circuits
and, more specifically, methods and apparatus for improved ESD protection
in CMOS integrated circuits.
BACKGROUND OF THE INVENTION
Semiconductor integrated circuits are quickly destroyed when subjected to
excessive voltages. One of the most common causes of damage is
electrostatic discharge or ESD. An ESD event occurs whenever a packaged IC
is subjected to the dissipation of static electricity, which may occur
whenever the pins of the IC come into contact with another surface. Thus,
the likelihood of an ESD event damaging or destroying an IC is substantial
during packaging and handling of the IC. Even after an integrated circuit
is mounted on a circuit board and housed within a system, such as a modem
or PC, it is nonetheless susceptible to ESD events discharging in and
around the circuitry.
The human body is a major source of static charge. It is sometimes modeled
as a 100 picofarad capacitor, capable of storing two or three kilovolts
and having a series resistance on the order of a few K-ohms. Thus, when
the pins of a packaged integrated circuit are touched by a person, a peak
current on the order of two amperes can be delivered through the MOS
devices on the IC. These voltages and currents can easily damage or
destroy the gate oxides of modern MOS devices on the IC which have
sub-micron geometries. To address this problem, most ICs are provided with
some sort of ESD protection scheme.
Frequently, ESD protection schemes comprise one or more diodes or SCR
circuits coupled between each input/output (I/O) pad on the chip, and the
power supply rails. When an excessive voltage appears at the corresponding
pin, for example an ESD event, the diode, SCR or similar circuitry turns
on very quickly, to short the high voltage to the power supply node. For
example, U.S. Pat. Nos. 4,829,350; 4,811,155; 4,855,620; and 4,692,834 all
disclose ESD protection circuits in which the channel of an MOS device is
coupled between ground and a pin of the IC. Such an MOS device has a
parasitic bipolar transistor coupled in parallel with the MOS device. When
a positive ESD event occurs at the pin, the bipolar transistor is
forward-activated, and a substantial portion of the ESD current is passed
through it to ground. An electrostatic discharge protection circuit having
a non-lightly doped drain MOS device for protecting other lightly doped
drain devices is disclosed in U.S. Pat. No. 5,246,872. A method for
forming a silicon-controlled rectifier (SCR) in a semiconductor integrated
circuit is described in U.S. Pat. No. 5,369,041. Another CMOS on-chip ESD
protection circuit and related semiconductor structure are shown in U.S.
Pat. No. 5,182,220.
Typically, the IC "chip" or die includes an I/O "slot" adjacent each
bonding pad. Each slot is simply a predetermined area in which a selected
I/O circuit, such as a buffer circuit, is formed when the rest of the chip
is fabricated. Standard buffer circuits or "cells" are inserted from a
cell library as needed. These buffer circuits generally include an ESD
protection structure, as noted, but in any event the buffer must fit
within the standard slot size. Since the level of ESD protection or
immunity depends upon silicon area of the protection structure, the level
of protection is limited by the I/O slot size on the die. This level of
protection is often inadequate.
Moreover, the present inventor has observed that corner pins of an IC are
adversely affected by ESD more often than the other pins. This is
apparently due to the tendency of a high-voltage discharge to concentrate
at the corners of the package. What is needed therefore is to improve ESD
immunity in integrated circuits, without increasing silicon area dedicated
to the task. What is also needed is to improve immunity particularly at
corner pins, again without increasing silicon area.
SUMMARY OF THE INVENTION
In view of the foregoing background, one aspect of the invention is
embodied in a semiconductor integrated circuit product. Such an IC product
includes a circuit chip or die containing core electronic circuitry and
having a plurality of I/O pads, at least one of the pads being used for
electrical connection to the core electronic circuitry and at least
another one of the pads being unused. At least two buffer structures are
formed on the circuit chip (as in prior art), one of the buffer structures
being connected to the used one of the pads and the other buffer structure
being connected to the unused one of the pads. The buffer structures each
includes an ESD protection structure for protecting core electronic
circuitry connected to the corresponding pad from excessive voltage
appearing at a corresponding external pin. Finally, a conductor is formed
on the IC interconnecting the used pad and the unused pad, thereby
interconnecting the corresponding buffer structures to one another in
parallel so as to improve ESD protection of the core circuitry connected
to the used pad. In one example of such a product, the plurality of pads
are located along a peripheral edge of the chip; and the used pad and the
unused pad are located adjacent one another along the peripheral edge of
the chip. In another example, the chip includes pads arranged along
generally rectilinear edges of the chip including first and second
adjacent edges forming a corner of the chip; and the used pad is located
adjacent the corner of the chip. In this way, additional protection is
provided for the corner pad.
According to another aspect of the invention, a method of improving ESD
protection of core circuitry connected to a first I/O pad in a
semiconductor integrated circuit comprises providing a buffer circuit
having an ESD protection structure connected to the first I/O pad;
identifying an unused pad on the IC that is not connected to the core
circuity but includes an ESD protection structure connected to the pad;
and electrically connecting the unused pad to the first pad, thereby
connecting the corresponding ESD protection structures to each other in
parallel, so that improved ESD protection is provided to the first I/O pad
without adding additional circuitry to the chip. Typically the connection
would be provided by a metallization layer formed within the usual
fabrication process.
Still another aspect of the invention can be summarized as an improved
method of designing an ASIC for improved ESD immunity. It is assumed the
ASIC includes an array of I/O pads generally for connecting core circuitry
to external package pins, each I/O pad including an I/O buffer, and each
I/O buffer including an ESD protection structure. The improved method
calls for first obtaining an initial design of the ASIC including a
specification of I/O connections; determining which of the I/O pads in the
array are not used for connection to the core circuitry according to the
initial design; and then modifying the initial design so as to provide
interconnection of at least one of the unused pads to a selected pad in
the array that is used for connection to the core circuitry, thereby
enhancing ESD protection of the core circuitry connected to the selected
pad. The method is advantageously utilized especially where the selected
pad is adjacent a corner of the ASIC chip.
The foregoing and other objects, features and advantages of the invention
will become more readily apparent from the following detailed description
of a preferred embodiment which proceeds with reference to the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a top plan view of a packaged integrated circuit product.
FIG. 2 is an enlarged, top plan view of a corner region of an integrated
circuit die illustrating one illustrative embodiment of the present
invention.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT
FIG. 1 is a top plan view of a typical integrated circuit product. By
"product" we mean that the integrated circuit chip is packaged and ready
for shipment to a customer. The product 10 thus includes an integrated
circuit chip (not shown) housed in packaging material 12 such as a
polymeric material. The package 12 includes a plurality of pins, for
example pins 14 and 16 extending outwardly around the periphery of the
package according to one of various industry standard pin configurations.
The pins are arranged for insertion into a corresponding socket, or
surface mounting, for connecting the product 10 to other electronic
circuitry. As is known in the prior art, many of the pins are electrically
connected via bonding wires to I/O pads on the integrated circuit die.
Other pins are connected to power and ground planes which in turn are
connected to corresponding power and ground pads on the die. Finally, FIG.
1 points out corner pins on the IC package. As noted previously, circuitry
connected to corner pins tend to be more susceptible to ESD events as the
high voltage discharge is concentrated toward the corners of the package.
By "corner pins" we mean not only the pins immediately adjacent to the
corner of the package (two pins at each corner, oriented orthogonally),
but also including additional pins which are nearby but not immediately
adjacent to the corners of the package.
FIG. 2 is an enlarged, top plan view of a corner region of an integrated
circuit chip or die 20 illustrating one embodiment of the invention. The
chip 20 includes a plurality of I/O slots 22, 24, 26, 28, 30 and 32. As
described in the Background, each of the I/O slots has a predetermined,
fixed size, for forming an I/O circuit, such as an I/O buffer circuit.
Each I/O slot buffer circuit is connected via a corresponding conductor,
for example conductor 36, to a corresponding bonding pad 42, 44, 46, 48,
50 and 52, respectively. According to the prior art, all of the I/O slots
are of a consistent size, and thus no special consideration is given to
enhancing ESD protection for any particular bonding pad.
In the die 20, some of the bonding pads, namely 44 and 50, are unused in
the sense that they are not connected to internal core circuitry on the
chip. Nonetheless, the corresponding I/O slots 24, 30 include ESD
protection circuits as they are generally provided for all I/O pads
regardless of whether the pads are used or not. According to the
invention, these unused buffer circuits, which include ESD protection
structures, are employed to include ESD protection of circuitry connected
to other I/O pads that are used. So, by way of illustration, a conductive
material, typically metal, is formed so as to interconnect a used pad and
an unused pad, thereby interconnecting the corresponding buffer structures
to one another in parallel, so as to improve ESD protection of the core
circuitry connected to the unused pad. Thus, in FIG. 2, a conductor 56 is
provided for interconnecting the used pad 46 to the unused pad 44.
Similarly, another conductor 60 is provided so as to interconnect the used
pad 48 to the unused pad 50. As a result, the used pads 46 and 48 enjoy
increased ESD protection by way of the ESD protection structures provided
in buffer circuits 24 and 30, respectively. This provides improved ESD
immunity at the corner pads which are most susceptible. This invention is
useful for any IC design, but is especially beneficial for "core-limited"
designs in that no additional I/O circuitry is needed, nor is additional
silicon area required. In "corelimited" designs, there are extra I/O pads
(and buffer circuits) that are available. Since the ESD protection is
increased as the amount of "area" that an ESD charge "sees" during
electrostatic discharge through a pin, any increase in the area improves
the protection. The present invention is expected to nominally double the
protection by essentially providing two ESD protection structures in
parallel.
The invention is not limited to protection of corner pins. It can just as
well be applied to increase protection of pins anywhere in the package.
Moreover, the inventive concept is not limited to connecting a single
unused pad to a used pad. More than one unused pad can be connected to a
used pad for even further protection. Preferably, unused pads are
connected to neighboring used pads for this purpose. However, in some
applications, it may be necessary to interconnect pads that are not
immediately adjacent. The connections described, e.g., 56 and 60, are
provided during the metalization step of the usual fabrication process.
Thus, no changes to the standard IC fabrication process are required. The
invention is applicable to a wide array of integrated circuits,
particularly CMOS circuits, and including Application Specific Integrated
Circuits ("ASIC") as well as full custom designs.
To summarize, the present invention provides additional ESD protection
without occupying additional silicon real estate, without requiring new or
special ESD structures for corner pins or other more susceptible pins.
Moreover, the present invention can be easily implemented into existing
layout tools.
Having illustrated and described the principles of my invention in a
preferred embodiment thereof, it should be readily apparent to those
skilled in the art that the invention can be modified in arrangement and
detail without departing from such principles. I claim all modifications
coming within the spirit and scope of the accompanying claims.
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